Nowadays, displays are omnipresent in everyday life. Various technologies can be implemented in displays, such as Light Emitting Diodes (LEDs), Liquid Crystal Displays (LCDs), Organic Light Emitting Diodes (OLEDs) or Plasma Displays for instance. Displays can either produce light themselves without any backlight layer or either they can need an extra source of light, the so-called a backlight. LCDs belong to this second category as they need an illumination source—the backlight—to produce a visible image. Usually, a LCD is made up of liquid crystals which are arrayed in front of the backlight, of two transparent electrodes and of two polarising filters. By controlling the voltage applied across the liquid crystal layer in each pixel, light provided by the backlight can be allowed to pass through in varying amounts thus constituting different levels of gray. It should be noted that a pixel corresponds to a certain LCD surface.
The light source of a backlight can be made up of various sources such as OLEDs, Quantum Dots or phosphors. Most commonly, they are made up of one of several LEDs, for instance Red, Green and Blue (RGB) LEDs. Usually, the backlight is designed to emit a white light. By controlling the light emitted by each LED, one can change the brightness and “colour point” of the backlight. By “colour point”, it should be understood the coordinates of the colour in the CIE 1931 xy chromaticity diagram.
The brightness and colour point of LEDs backlight can vary based on a number of conditions. For instance, a change in temperature or the ageing of LEDs can have strong impacts on the brightness level and colour point of LEDs. For certain applications, those changes are not acceptable. In a television for instance, the colour point of a backlight should be as stable as possible, in order to produce images as accurate as possible. In avionics, displays provide critical flight information to aircraft pilots. Such displays should be readable under a variety of lighting conditions.
Generally, backlight LEDs are controlled by Pulse Width Modulated (PWM) signals. In order to ensure the readability of displays, notably in avionics, but not only, a dynamic control of LEDs has been implemented in prior art and feedback control loops have been provided to stabilize the LEDs features. After measuring the LED temperature and the luminous flux of LEDs, PWM controllers can adjust PWM signals sent to LEDS to maintain the desired colour point and brightness level of LEDs.
Two main drawbacks remain to be overcome in LEDs backlights.
The first drawback is to reduce combined peak current. Combined peak current occur notably when PWM signals are the same for all LEDs, i.e. when all LEDs are switched ON and OFF respectively at the same time. This phenomenon is illustrated in FIG. 1 showing three PWM signals for three LEDs, respectively, with the function of combined current in the three LEDs. Peaks of power consumption are often created, involving issues of noise and electromagnetic compatibility. The peak current has influence on the power system. Big step loads make the power supply more complex and bigger. The induced effects of peak currents are even more problematic when large displays are used. The larger the displays, the bigger will be these step loads and the more problematic will be the induced effects. Indeed, when designing a display, specifications can be given by client, such as for instance a maximum of 5% of power modulation on the nominal power.
This means that for a nominal power of 50 W, the power consumption can vary between 47.5 and 52.5 W. The power required to illuminate large displays being higher than for small displays, by keeping the same specifications, this would create larger range of modulation. For a nominal power of 100 W and the same specification, the range of accepted values would be 95 to 105 W. This is not acceptable for clients who want to maintain the brightness of displays as stable as possible, and thus reduce the range of acceptable variation.
The second drawback concerns the reliability of luminous flux measurements. In a typical RGB backlight, a plurality of optical sensors can be used to measure the brightness of LEDs. Each sensor can be dedicated to a given colour. Unfortunately, the sensitivity of sensors being usually broad, an overlap between sensitivity spectrums can occur, as illustrated by FIG. 2. When measuring the brightness of a given colour, for instance blue, the sensor can measure the brightness of both blue and green LEDs, and the measure can be biased.
In prior art, several methods have been presented to optimize the feedback control loops by adjusting the PWM signals.
WO 2012/140634 discloses PWM signals which are phase-shifted in order to reduce combined peak current provided to the light sources, as illustrated in FIG. 3.
U.S. Pat. No. 8,175,841 describes a method for controlling an illumination system, according to which measurements of luminance are carried out by a single full spectrum optical sensor when only one single colour is switched on, in order to avoid measuring a biased colour point. Unfortunately, this method involves instability in power consumption i.e. combined peak current during the measure of colour points, as only one colour is switch on during this phase.
Despite what has been presented in prior art, a method remains to be proposed in order to measure non-biased colour points while keeping stable power consumption.